I recently started a book about Romania in the First World War and came across all the major events in there being signaled with different dates (15/28 June 1914, assassination of Franz Ferdinand). Normally I thought this was an error in typing, but the same thing is repeated for almost every date in the book. I was unable to find the reason why for this and so I decided to mark it off as some editing error. However, I was browsing Wiki recently and came across the following: "The Second Balkan War broke out on 29(16) June 1913 when Bulgaria attacked its erstwhile allies in the First Balkan War". Never have I seen this before in any other books or videos.So why is this happening? What is the reason for this?